The present invention relates to a reflection type liquid crystal display device using a liquid crystal cell for controlling light in a scattered or transparent state.
In a conventional, liquid crystal display device, a polymer-dispersed liquid crystal display element or a scattering/transparent liquid crystal display element such as a liquid crystal cell has been developed, because the liquid crystal cell has a simpler structure than that of a TN type liquid crystal cell, can be manufactured easily, and utilizes light at a higher rate without using a polarizing plate. In the polymer-dispersed liquid crystal display element, a liquid crystal layer in which a polymeric material and a liquid crystal are dispersed is sealed between a pair of substrates respectively having transparent electrodes.
In this polymer-dispersed liquid crystal display element, in the electric field OFF mode wherein no voltage is applied across a pair of transparent electrodes, the liquid crystal molecules are aligned in a random manner in the liquid crystal layer to exhibit a scattered state. In the electric field ON mode, the liquid crystal molecules are aligned in the direction of electric field in the liquid crystal layer to exhibit a transparent state.
To perform monochromatic display with this liquid crystal display device, an optical element such as a black light absorber, a scattering reflection plate, or a specular reflection plate is arranged on the rear side of the polymer-dispersed liquid crystal cell. In the electric field OFF mode wherein no voltage is applied across the electrodes of the liquid crystal cell, the liquid crystal cell exhibits the scattered state. When the user observes scattered light, white display can be obtained. In the electric field ON mode wherein a voltage is applied across the electrodes of the liquid crystal cell, the liquid crystal cell at a portion applied with the electric field exhibits the transparent state. The optical element arranged on the rear side of the liquid crystal cell can be observed directly to be seen black. Alternatively, the user may observe weak reflected light or may not observe reflected light, so that the portion applied with the electric field is displayed in black.
In this liquid crystal display device, if a black light absorber is used as an optical element to obtain the darkest black display, although black display can be assured, bright white display cannot be obtained. This is due to the following reason. When light passing through the liquid crystal layer is scattered, it produces backward scattered light directed opposite to the propagating direction of light passing through the liquid crystal cell, and forward scattered light directed along the propagating direction of light coming incident on the liquid crystal cell. Of the backward and forward scattered light, the forward scattered light, i.e., light directed toward the rear surface of the liquid crystal cell, is absorbed by the light absorber.
In a liquid crystal display device using a scattering reflection plate as the optical element, in the electric field ON mode, light transmitted through the liquid crystal cell is scattered by the scattering reflection plate, and part of the scattered light is transmitted through the liquid crystal cell again and is observed. Hence, sufficient black display cannot be assured.
In a liquid crystal display device using a specular reflection plate as the optical element, a high contrast is obtained under specific observation conditions. However, due to the specular properties of the reflection plate, the observer himself is reflected in the reflection plate, black display glitters, or the range of a viewing angle is narrow.
It is an object of the present invention to provide a reflection type liquid crystal display device in which bright white display and dark black display are assured to improve the contrast, so that a wide visual angle can be obtained.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a reflection type liquid crystal display device using a liquid crystal cell for controlling light in a scattered or transparent state, comprising: a scattering/transparent liquid crystal cell having a liquid crystal layer for controlling incident light in the scattered or transparent state in accordance with an applied electric field; and a selective reflector arranged on a side of the liquid crystal layer opposite to a side where observation is performed, and having reflecting characteristics to cause incident light, which has come incident from a direction within a predetermined range of a viewing angle that allows observation of the liquid crystal cell, to emerge in a direction outside the range of a viewing angle.
In this liquid crystal cell device according to the first aspect, either a scattered state or a transparent state is selected for light that has passed through the liquid crystal layer (to be referred to as transmitted light hereinafter) upon application of an electric field. When the transmitted light is in the transparent state, light coming incident within the range of a viewing angle can be caused to emerge in a direction outside the range of a viewing angle using an incident angle at which the transmitted light comes incident on the selective reflector, so this light is not observed. Thus, dark display can be obtained from the observing direction. When the transmitted light is in the scattered state, the scattered light emerges at an angle within the range of a viewing angle. Thus, bright display can be obtained.
In the liquid crystal display device according to the first aspect, the selective reflector has a light incident surface that forms prisms on which a plurality of slants inclined in opposite directions are aligned, and a specular reflection layer formed on at least one of two slants of the plurality of slants.
In the liquid crystal display device using this selective reflector, its exit direction can be adjusted by the angles of the slants and the incident angle. The emerging direction of light coming incident at an angle within the range of a viewing angle can be set outside the range of a viewing angle.
In the liquid crystal display device according to the first aspect, the selective reflector may have a first optical layer where light-transmitting portions and light-absorbing portions are aligned alternately, and a second optical layer where light-absorbing portions and light-reflecting portions are aligned alternately, the first and second optical layers opposing each other at a predetermined distance, while the light-transmitting portions of the first optical layer and the light-absorbing portions of the second optical layer are set to correspond to each other and the light-absorbing portions of the first optical layer and the light-reflecting portions of the second optical layer are set to correspond to each other.
In the selective reflector of this liquid crystal display device, light, coming incident on it from near the direction of normal to the front surface of the selective reflector, i.e., light from a direction within the range of a viewing angle, can be absorbed by the respective light-absorbing portions formed on the first or second optical layer. Light inclined from the direction of normal, and light coming incident at an angle outside the range of a viewing angle, can be transmitted through the light-transmitting portions of the first optical layer, be reflected by the light-reflecting portions of the second optical layer, and be transmitted through the light-absorbing portions of the first optical layer, to emerge from the front surface of the selective reflector.
Transmitted light in the transparent state, which has come incident at an angle within the range of a viewing angle, does not emerge from the front surface of the selective reflector. Hence, dark display can be obtained. Transmitted light in the scattered state emerges from the front surface of the selective reflector, so that bright display can be obtained. Since this selective reflector has a simple structure, it can be easily manufactured from the selective reflector having the prism structure described above.
According to the second aspect of the present invention, there is provided a reflection type liquid crystal display device using a liquid crystal cell for controlling light in a scattered or transparent state, comprising:
a scattering/transparent liquid crystal cell having a liquid crystal layer interposed between a pair of substrates to control incident light in the scattered or transparent state in accordance with an applied electric field; and
a selective reflector arranged on a side of the liquid crystal layer opposite to a side where observation is performed, and having angle-selective reflecting characteristics to transmit first incident light, coming incident at a first angle range within a predetermined range of a viewing angle to allow observation of the liquid crystal cell, and to reflect second incident light, coming incident within outside the range of the first range of viewing angle, in a direction outside the first range of a viewing angle, of light passing through the liquid crystal layer, the selective reflector transmitting the first incident light coming incident within the range of a viewing angle toward a rear surface of the device, and reflecting the second light coming incident outside the range of the first viewing angle toward the liquid crystal layer.
In the liquid crystal cell device according to the second aspect, either a scattered state or a transparent state is selected for light that has passed through the liquid crystal layer upon application of an electric field. When the transmitted light is in the transparent state, light coming incident at an angle within the range of a viewing angle can be caused to emerge in a direction toward the rear surface of the selective reflector by an incident angle at which the transmitted light comes incident on the selective reflector, so this light is not observed. Thus, dark display can be obtained from the observing direction. When the transmitted light is in the scattered state, the scattered light emerges at an angle within the range of a viewing angle. Thus, bright display can be obtained.
In this liquid crystal display device, the selective reflector has a light-incident surface that forms prisms on which a plurality of slants inclined in opposite directions are aligned, a specular reflection layer formed on one of two slants of the plurality of slants, and a transparent surface formed on the other one of the two slants, and
the device has a surface light source unit constituted by a light source, having an exit surface opposing a rear side of the device, and a light guide.
In this liquid crystal display device, a specular reflection layer is formed on some slants of the selective reflector, and the remaining slants are formed transparent. Since the surface light source unit is formed on the rear side of the selective reflector, in addition to light coming incident from the front surface of the liquid crystal cell, light emerging from the surface light source unit can be transmitted through the transparent surface formed on the selective reflector, and can be utilized as back illumination light. Thus, this liquid crystal display device can be used at a dark place as well.
According to the third aspect of the present invention, there is provided a reflection type liquid crystal display device using a liquid crystal cell for controlling light in a scattered or transparent state, comprising:
a scattering/transparent liquid crystal cell having a plurality of thin films formed on at least one of a pair of substrates, and a liquid crystal layer interposed between the pair of substrates, to control incident light in the scattered or transparent state in accordance with an applied electric field; and
a selective reflector arranged outside one of the substrates which forms a rear surface of the liquid crystal cell, and having a reflecting surface having a predetermined angle of inclination so that light, coming incident on the liquid crystal cell outside the range of a viewing angle, comes incident on an either one of interfaces among the plurality of thin films and the pair of substrates at an incident angle satisfying a total reflection condition which prevents light to emerge toward a front surface of the liquid crystal cell.
With the liquid crystal display device according to the third aspect, light coming incident at an angle outside the range of a viewing angle, which is largely inclined with respect to the normal to the front surface of the liquid crystal cell, is caused by the selective reflector to be totally reflected by either one of the interfaces of the plurality of thin films and the pair of substrates, and does not emerge toward the front surface of the liquid crystal cell. Since unnecessary light does not emerge, when the liquid crystal cell is in the transparent state to transmit light passing through it, it decreases unnecessary exit light to obtain dark display, thereby improving the contrast.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.